U.S. Pat. No. 4,906,349 discloses a known generic probe. This probe is part of a measuring head that is to be immersed into a metal melt. A small tubelet protruding out of the measuring head is made out of stabilized zirconium oxide which when used at high melt temperatures (about 1500° C. up to 1800° C.) is predominantly oxygen conducting and neglectably electron conducting. The small tubelet is closed at the protruding end and located therein is a reference substance in the form of a powdery mixture of chrome and chrome oxide, which is connected with a measuring instrument by an electric conductor. The measuring instrument is furthermore connected to the outer surface of the tubelet via an electron conducting contact with the metal melt (bath) and further via the metal melt and therefore measures the potential difference between the outer and inner surface of the tubelet, so that, having regard to the temperature, the oxygen activity in the melt can be calculated. The oxygen activity corresponds to the oxygen content dissolved or otherwise present in the respective substance, for example a metal melt, particularly a steel melt.
The principle of the measurement of the oxygen activity with such probes is also disclosed in the article “Sauerstoffmeβsonde FOX für Stahlschmelzen” in the journal “Stahl und Eisen” 95 (1975), No. 22, page 1084.
From U.S. Pat. Nos. 3,752,753 and 3,773,641 it is known that just a plug made from a solid electrolyte may be cemented into the open end of a small refractory tubelet. The plane outer front side of the plug is in contact with the melt during the measurement.
Finally, embodiments are known where a solid electrolyte is in the form of a coating on a carrier pin or on a small carrier tubelet, as can be seen in DE 28 33 397 A1. These probes have a particularly small reaction time, as the thickness of the coating is below 300 μm, and the thermal and electric equilibrium is soon reached.
Probes of the described type have become standard as measuring elements for the oxygen activity especially for monitoring the progress of the deoxidation process with aluminum. Their function is quite satisfactory in the range of higher oxygen levels, that is from 100 ppm to 1000 ppm or more.
However, it was noticed that the reliability, accuracy and especially the reproducibility of the measurement is unsatisfactory in the range of low oxygen levels from 1 ppm to 100 ppm.
The accuracy problem is also dealt with in the already mentioned U.S. Pat. No. 4,906,349. According to this document, the probes to be immersed in the melt are mechanically cleaned on their outer surface by etching or sand blasting in order to allow measuring at oxygen concentration from zero to 20 ppm with a higher precision and with an improved response time.
The mechanical and chemical cleaning of the surface pointing into the melt of a solid electrolyte like zirconium oxide requires an considerable additional expenditure, because it is a very hard and resistant material.
One of the objects of the invention is to improve functionality of the probes at low oxygen concentrations without further expenditure and without risk of unreliable measurements.